Pivots & Collars
In ships, steam and water turbines etc. by the very nature of mechanism of forces developed in them, their shafts are subjected to axial force, which is known as axial thrust. This naturally, produces axial motion of the shafts. In order to prevent it and preserve the shaft in correct axial position, they are provided with one or more bearing surfaces at right angle to the axis of shaft. A bearing surface provided at the end of a shaft is known as a pivot and that provided at any place along with the length of the shaft with bearing surface of the revolution is known as collar. Pivots are of two forms: flat and conical. The bearing surface provided at the foot of a vertical shaft is called footstep bearing.
Due to the axial thrust conveyed to the bearings by the rotating shaft, rubbing takes place between the contacting surfaces. This produces friction as well as wearing of the bearing.
Thus work is lost in overcoming the friction, which is ultimately to be determined under this article. Obviously the rate of wearing depends upon the intensity of thrust and elative velocity of rotation.
\rate of wear µ p x r
Now there could be two assumptions on which we can proceed further:
Firstly, the intensity of pressure is uniform over the bearing surface. This assumption only holds good with newly fitted bearings where fit between the two contacting surface is assumed to be perfect. As the shaft has run for sometime the pressure distribution will not remain in uniform due to varying wear at different radii.
Secondly, the rate of wear is uniform. The rate of wear is proportional to p x r as we have
already discussed which means that the pressure will go on increasing radially inward and at the center where r=0, the pressure must be infinite which is not true. Hence this assumption too, has fallacies and anomalies. However, the assumption of wear gives better practical results. The various types of bearings mentioned above will be dealt which separately for each assumption.